Sawmills and lumber mills typically route lumber through a series of cutting and finishing stations. After exiting one station, the lumber must be positioned for presentation to the next station. This positioning may take place during transportation of the lumber from one station to the next. In certain stations, boards are trimmed at one or both ends. Current systems often position boards lengthwise along a conveyor with evenly spaced lugs used to separate and convey the boards towards the trimming station.
Conventional positioning systems often use conveyor chains to carry boards positioned lengthwise across the width of the conveyor, while simultaneously using pipe rolls or similar mechanisms to urge the boards transverse to the direction of flow towards a fence that results in a workpiece position where the defect line matches a downstream saw position. Other workpiece positioning systems do not use the transverse rolls, but rely instead on a positioning bumper to push wood into position (see for example U.S. Pat. No. 591,130).
In moving paddle fence systems, boards are pushed against positioning bumpers or “paddles” connected to chains that are configured in a continuous moving loop. The paddles move in the direction of material flow, parallel with the board conveyor chains and at the same velocity. The rotating chain and paddle concept allows more time for each board to reach its final destination than a traditional trailing fence design.
Speed and accuracy are critical in positioning lumber for presentation to sawing stations—inaccurate positioning leads to waste, while slower speeds limit output. However, in the current positioning systems, increasing the processing speed tends to cause a reduction in accuracy. Similarly, as the current systems are adapted to increase accuracy, speed is diminished. As planer optimization becomes more widely accepted, the need for positioning accuracy has increased. But lumber system speeds have increased to such an extent that the traditional trailing fences are not as effective as they once were, often resulting in the positioning systems causing a bottle neck in the processing system.
Existing paddle fence systems use mechanisms such as paddle brakes, air actuators, and/or hydraulic positioners to position paddles. These mechanisms increase the accuracy of positioning. However, they also increase the complexity of the systems, add mass to the paddles, induce vibration and increase friction. Therefore, existing systems are limited in their range of possible operation speeds relative to lug spacing and physical size. To match the wider lug spacing configuration frequently used in sawmill board transfers while also matching the board transfer speeds, the currently available paddle fence systems must be physically extended to compensate for the vibration and other problems created by the positioning mechanisms.